Optical apparatus

Optical trackers of the kind which are used as position detectors and which incorporate Fourier transform filters 6, 7 matched to an object 1, 2 to be detected within a field-of-view, e.g. a target 2, indicate the position of object within the field-of-view if it is present as the position of a spot of light in the correlation plane 8 of the tracking system. However, positional errors may be introduced due to inaccuracies in the alignment of the filter axis and the reconstruction lens used to provide the output and absolute positions cannot be obtained. The invention described herein uses a Fourier transform filter 6 matched to two objects which may appear together in a given field-of-view e.g. missile and target, the presence of both objects in the field-of-view being indicated as two spots of light in the correlation plane of the tracker, the relative positions of which being equivalent to the relative positions of the two objects within the field-of-view, which is preceded by a device 4 which spatially modulates the incoming incoherent light onto a laser beam.